One disclosure of the present specification provides a user equipment method. The method comprises the steps of: receiving a time division duplex (TDD) uplink (UL)-downlink (DL) configuration from a frequency division duplex (FDD)-based primary cell; receiving downlink data from at least one secondary cell; determining an uplink subframe of the primary cell to transmit a hybrid automatic repeat request (HARQ) acknowledgement (ACK)/negative-acknowledgement (NACK) for the received downlink data, which is transmitted from the at least one secondary cell, according to the TDD UL-DL configuration; and transmitting the HARQ ACK/NACK from the determined uplink subframe to the primary cell.